The present invention relates to a method of forming a porous film and a material for a porous film used in the method.
As conventional methods of forming a porous film, the following four methods are known: In a first method, which is disclosed in "IEEE Transactions on components, hybrids, and manufacturing technology" (Vo. 15, No. 6, p. 925 (1992)), an organic polymer film is first formed from a copolymer of an organic polymer precursor with high thermal stability and an organic polymer precursor with low thermal stability, and a portion of the organic polymer film corresponding to the organic polymer precursor with low thermal stability is decomposed by heat treating the organic polymer film, thereby forming a porous film. In a second method, which is disclosed in "Advanced materials for optics and electronics" (vol. 1, p. 249 (1992)), an organic porous film is formed by depositing an organic polymer from an organic polymer solution placed in a supercritical state attained under a high pressure. In a third method, which is disclosed in "Macromol. Chem., Macromol. Symp." (42/43, 303 (1991)), an organic polymer containing silica film is first formed from a mixed solution of a silanol sol and an organic polymer, and the organic polymer is thermally decomposed by heat treating the organic polymer containing silica film, thereby forming a porous film. In a fourth method, which is disclosed in "Conference Proceedings of Advanced Metallization and Interconnect Systems for ULSI Applications in 1995, 29", a porous film is formed by gelating a silica sol by the sol-gel processing with controlling dryness of the silica sol. The fourth method is described in detail in Japanese Laid-Open Patent Publication Nos. 7-321206 and 8-162450. In addition, a fifth method is also known, as disclosed in Japanese Laid-Open Patent Publication Nos. 6-283864 and 8-330300, in which an organic material is dispersed in a silica coating film, and the organic material included in the silica coating film is blown through photochemical or thermal decomposition, thereby forming a porous film.
However, the first method of forming a porous film, in which the organic polymer film of the copolymer of the organic polymer precursor with high thermal stability and the organic polymer precursor with low thermal stability are heat treated so as to decompose the portion corresponding to the organic polymer precursor with low thermal stability, has a problem that the heat treatment is required to be conducted for 9 hours at a temperature of 275.degree. C.
Also, the second method of forming a porous film by depositing the organic polymer from the organic polymer solution in the supercritical state has a problem that a pressure as high as several hundreds atomospheric pressure is necessary to attain the supercritical state.
Also, in the third method of forming a porous film by forming the organic polymer containing silica film from the mixed solution of silanol sol and the organic polymer and thermally decomposing the organic polymer included therein, the heat treatment is required to be conducted for 24 hours at a temperature of 600.degree. C. Therefore, this method is very difficult to adopt, from a practical viewpoint, in formation of an interlayer insulating film of a porous film in manufacture of a semiconductor device.
Furthermore, the fourth method of forming a porous film by gelating silica sol by the sol-gel processing with controlling the dryness of the silica sol has a problem that an aging process is required to be conducted in an atmosphere of a solvent for controlling the volatizing rate of the solvent of the silica sol.
In addition, the fifth method of forming a porous film by blowing the organic material included in the silica coating film through photochemical or thermal decomposition is disadvantageously complicated.
Accordingly, the present inventors have proposed a method of forming a porous film in Japanese Laid-Open Patent Publication No. 9-298241. In this method, a residual silanol group in silanol condensate particles is chemically modified by a silyl group by adding a silylation reagent to a solution including silanol condensate particles (a commercially available SOG solution), and the solution including silanol condensate particles is applied on a substrate so as to form a coating film. The coating film is then heat treated so as to thermally decompose the silyl groups chemically modifying the silanol condensate particles. Thus, a porous film is formed.
Since this method of forming a porous film utilizes a SOG process generally used in the manufacture of a semiconductor, it is superior to the aforementioned conventional methods in obtaining a porous film through a simple process.
In order to minimize the size of a pore included in a porous film to an extent required in an interlayer insulating film used in an LSI, it is necessary to add the silylation reagent to the solution including silanol condensate particles in the presence of, for example, amine. However, when amine is added to the solution including silanol condensate particles, there arises a problem that the property of the solution including silanol condensate particles (namely, the material for the porous film) is changed with time in a very short period of time.